1. Field of the Invention
The present invention relates to a control system and method for a wire insulating line, and more particularly to a control system and method for a wire insulating line for manufacturing a covered wire by covering a wire with an insulator composition of foamed synthetic resin.
2. Description of the Prior Art
A covered wire produced by a wire insulating line is used as a communication means for telephone equipment and general electrical equipment. A high precision of finish specifications of electric characteristics of the covered wire such as capacitance and an outer diameter is required.
On the other hand from a viewpoint of productivity, a high drawing speed of the covered wire is required, and it is also required to raise a drawing speed to higher than or equal to 2000 m/min in a short time after starting of the production line and to make the finished outer diameter and capacitance of the covered wire to fall within a tolerance range soon after starting of the production line.
FIG. 3 is a schematic diagram of such a wire insulating line 1 and FIG. 4 is a flowchart showing a conventional control method for the wire insulating line 1.
In the wire insulating line 1 shown in FIG. 3, a stock wire 4b stored in a coil stocker 12 is reduced in diameter by a drawing machine 10 and is supplied as a core (wire) 4a to an annealer 11, in which the wire 4a is annealed. On the other hand, a synthetic resin such as polyethylene resin and an organic foaming material are supplied to an extruder 3 and are forced into a crosshead 2 provided at the end of a cylinder (not shown) of the extruder 3 by means of a screw (not shown) built in the cylinder.
The polyethylene resin and the organic foaming material are heated to a predetermined temperature higher than or equal to a decomposing temperature of the organic foaming material by means of a heater (not shown) provided in the crosshead 2.
Then, the wire 4a which is maintained at a suitable temperature is introduced into the crosshead 2 of the extruder 3, and is covered with the polyethylene resin to form a covered wire 4 having the same shape as the inner shape of the crosshead 2. Then, the covered wire 4 is hauled in a direction shown by an arrow F by a haul off machine 9 and is immersed into water in a water-cooling trough 6, in which the covered wire 4 is solidified by cooling. Thereafter, a capacitance C of the covered wire 4 is measured by a capacitance monitor 7, and an outer diameter D of the covered wire 4 is then measured by an outer diameter monitor 8.
In the wire insulating line 1 mentioned above, variables such as rotational speed of the screw in the extruder 3, etc. are controlled in proportion to a haul off speed V of the haul off machine 9 to obtain desired values of the capacitance and the outer diameter of the covered wire 4 as shown in FIG. 4 (S01).
The capacitance C of the covered wire 4 is adjusted according to its measured value by using a moving trough 5 to move the water-cooling trough 6 away from the extruder 3 in the drawing direction (S02 and S03). In other words, the adjustment of the capacitance C is effected by changing a distance between the extruder 3 and the water-cooling trough 6 (i.e., changing a position L of the water-cooling trough 6 within a control range of L0 to L1) to thereby adjust a period of time from covering of the polyethylene resin to solidification thereof and hence control a degree of foaming of the polyethylene resin. Further, the outer diameter D of the covered wire 4 is adjusted according to its measured value by either automatically or manually changing a rotational speed N of the screw of the extruder 3 and a set temperature .theta. of the extruder 3 (Japanese Patent Laid-open No. Hei 3-236120, for example).
In this manner, the outer diameter D and the capacitance C of the covered wire 4 are controlled by adjusting a supply amount of the resin, a cooling rate of the covered wire 4, and a temperature of the resin supplied.
However, the above conventional control method for the wire insulating line 1 has the following problem.
A delay time and a response time associated with the control of the resin temperature are considerably longer than those associated with both the control of the resin supply amount and the control of the cooling rate of the covered wire (the times for the latter are of the order of second, whereas the times for the former are of the order of minute). Further, there is a limit to the controllable range (moving range) in the control of the cooling rate of the covered wire 4 by the movement of the water-cooling trough 6. Accordingly, when the required control of the cooling rate is outside the controllable range as may occur during starting of the production line or upon a change of surroundings, the product cannot possibly be taken out until the foaming degree of the resin falls within a proper range under the control of the resin temperature.